Automatically generate static file names

[GuillaumeGomez]

Based on #107531 so needs it to be merged first.

In #107354, we were able to see that being able to correctly update the static files is not as simple and straight-forward as it could be. So instead of forcing the devs to do the work manually, this PR allows it to be done automatically to prevent such errors to occur again.

r? @notriddle

[rustbot]

Some changes occurred in HTML/CSS/JS.

cc @GuillaumeGomez, @Folyd, @jsha

[GuillaumeGomez]

Rebased so now only the second commit remains.

[notriddle]

I'm not entirely convinced that this is a good idea. It makes updating the font files more convenient, but one workflow I tend to use has me copying the rustdoc.css file into a compiled docroot to test changes to it without having to run the compiler again. This would break that.

Especially since the GUI tests will flag an error if someone fails to update the font files, and tspiteri added a comment explaining how to fix it, I'm not sure if this is all that important.

[GuillaumeGomez]

I'm not entirely convinced that this is a good idea. It makes updating the font files more convenient, but one workflow I tend to use has me copying the rustdoc.css file into a compiled docroot to test changes to it without having to run the compiler again. This would break that.

You convinced me with this argument. Let's keep the possibility to copy-paste the file directly. As you mentioned, the CI will fail in any case.